The invention relates to an asymmetrically driven audio-amplifier comprising a first amplifier circuit having an input and an output, a second amplifier circuit having an input and an output, first output terminals for connecting a first loudspeaker to the output of the first amplifier, second output terminals for connecting a second loudspeaker to the output of the second amplifier, a capacitor arranged between a first point and ground, the first output terminals being arranged between the output of the first amplifier circuit and the first point, the second output terminals being arranged between the output of the second amplifier circuit and the first point, and the first and the second amplifier circuits being adapted to carry signals of substantially opposite phase at their outputs.
Audio amplifier arrangements for use in conjunction with asymmetrical power supplies are inter alia employed in audio equipment intended for use in cars, such as car radios. In such amplifier arrangements a direct voltage of the order of half the supply voltage appears on the output of the amplifier arrangement. Since it is undesirable for a direct current to flow through a loudspeaker, a blocking capacitor is arranged between such a loudspeaker and the output of the amplifier arrangement in order to obtain d.c. isolation between the amplifier and the loudspeaker.
Audio-amplifier arrangements frequently comprise two amplifier circuits each having an input, an output and an associated set of output terminals for the connection to an associated loudspeaker. This is, for example, the case in stereo amplifiers, which comprise one amplifier and one loudspeaker connection per channel. It is also the case if, for each stereo channel, two loudspeakers are employed, for example one in the front of the car and one in the back of the car, the volume of the sound produced by said loudspeakers being variable relative to each other, for which purpose an amplifier circuit is provided for each loudspeaker. For d.c. isolation, a capacitor is then arranged between each set of output terminals and the corresponding amplifier output, which capacitors are electrolytic capacitors which are comparatively bulky.
U.S. patent application No. 4,186,273 describes such an audio-amplifier arrangement requiring only one blocking capacitor, which is arranged between a first point and ground, the first output terminals being arranged between the output of the first amplifier circuit and the first point and the second output terminals being arranged between the output of the second amplifier circuit and the first point.
In this way the output of the first amplifier circuit is connected to the output of the second amplifier circuit via the first loudspeaker, which is coupled to the first output terminals, and the second loudspeaker, which is coupled to the second output terminals, and the point between the two loudspeakers is at the same d.c. potential as the two outputs of the amplifier circuits, which d.c. potential also appears on the capacitor. In this way the two loudspeakers are d.c. isolated by means of one capacitor. The signal current which flows through the capacitor is small because the signal currents flowing through the two loudspeakers are substantially in phase opposition. Since the signal current through the capacitor is substantially smaller than in the case where one capacitor is used for each channel, it is possible to employ a capacitor of smaller dimensions than each of the separate blocking capacitors because the minimal dimensions of a capacitor depend on the ripple current flowing through this capacitor.